The present invention relates generally to transmission systems and more specifically to adaptive interleavers.
Interleaving is a coding technique that is commonly used to increase the performance of transmission systems by decreasing errors in the system. Interleaving rearranges the data that is to be transmitted in a given transmission thereby improving the error-correction performance of redundancy coding techniques. Interleaving increases the transmission latency of the interleaved transmissions. Latency is the time required for data to traverse the end-to-end transmission path.
In most applications, the latency associated with interleaving is only a small portion of the overall latency of the system. However, in telecommunications applications, and particularly with reference to digital subscriber lines, the latency associated with interleaving constitutes a significant portion of the overall latency. High latency can have a substantial negative impact on system performance especially when the system is operating at high data transmission rates. The impact is especially pronounced for systems where many end-to-end transmissions are required to accomplish a task, such as systems utilizing the popular TCP/IP data communications protocol to send a large file. Accordingly, telecommunications system providers generally strive to minimize latency throughout their systems while still utilizing interleaving to offset the adverse effects of errors. Thus, it is desirable to optimize the interleaving used such that only the degree of interleaving necessary to achieve a desired performance level is implemented.
Adaptive interleaving allows for different degrees of interleaving, commonly referred to as the interleave depth, to be applied to different transmissions. Adaptive interleavers are known to those skilled in the art. U.S. Pat. No. 4,901,319 describes an adaptive interleave system, including an adaptive interleaver, that attempts to correct errors that occur as a result of the fading characteristics of a radio channel. The system measures the phase error of transmissions in an effort to identify errors in the transmissions. The system utilizes a complex system and method to predict the next error occurrence based upon the measured phase error, and adjusts the adaptive interleaver in response to the prediction. However, measuring the phase error is not an effective method for identifying errors in many transmission systems. Also, a complex system for predicting the occurrence of errors and controlling an adaptive interleaver can be difficult to implement on many transmission systems.